Heat transfer systems include in particular refrigerators, heat pumps and conditioned air systems.
In such devices, a refrigerant with a suitable boiling point is evaporated at low pressure, taking heat from a first surrounding medium (or zone). The vapor thus formed is then compressed by means of a compressor and subsequently passes into a condenser, in which it is converted to the liquid state, giving rise to a release of heat into a second surrounding zone. The liquid thus condensed subsequently moves through a pressure-reducing valve, at the outlet of which it is converted to a two-phase mixture of liquid and vapor, which is finally introduced into the evaporator, where the liquid is again evaporated at low pressure, which completes the cycle.
The mechanical energy required to ensure the compression of the vapor and the circulation of the fluid is provided by an electric motor or an internal combustion engine. As in any mechanical device, it is necessary for the moving parts to be suitably lubricated. The lubricants used form an integral part of the heat transfer system and condition both its performance and its lifetime by the maintenance over time of suitable lubrication.
In particular, the refrigerant, which is at each pass through the compressor in contact with the lubricant present on its moving parts, tends to carry away a certain amount thereof, which accompanies the refrigerant in its cycle and is thus found in the evaporator. In point of fact, the latter is generally brought to a low temperature, at which the viscosity of the lubricant is particularly high, so that there is a risk of lubricant accumulating in the evaporator and thus the lubricant is no longer able to return to the compressor, this return being described in the present text as “oil return”.
Thus, if the oil return is inadequate, the amount of lubricant present on the moving parts of the compressor cannot be kept constant over time, which thus effects the satisfactory operation of said compressor and its lifetime.
It is therefore necessary to use a refrigerant/oil pair which is entirely compatible, in particular with regards to the oil return.
R-22 or monochlorodifluoromethane is a refrigerant of HCFC (HydroChloroFluoroCarbon) type widely used in heat transfer applications, including fixed air conditioning, commercial and industrial refrigeration, and heat pumps. There currently exist numerous heat transfer systems designed for R-22; the lubricants employed, as they are suitable for R-22, in particular as regards the oil return, are either mineral oils or alkylbenzene oils.
Although R-22 has a very low ozone depletion potential (hereinafter ODP), its use is, however, also subject to restrictions and novel products based on HFCs (HydroFluoroCarbons) have been developed which are particularly advantageous for the stratospheric ozone layer since HFCs exhibit a zero ODP.
Among these products, R-407C has in particular been developed for replacing R-22 in air conditioning applications. This product is a mixture combining R-32, R-125 and R-134a in the proportions of 23/25/52% by weight. R-32 is the usual name in the trade for difluoromethane, R-125 is pentafluoroethane and R-134a denotes 1,1,1,2-tetrafluoroethane. R-407C has thermodynamic properties which are very similar to those of R-22. For this reason, R-407C can be used in old systems designed to operate with R-22, thus making it possible to replace an HCFC fluid by an HFC fluid which is safer with regards to the stratospheric ozone layer in the context of a procedure for converting these old systems. The thermodynamic properties concerned are well known to a person skilled in the art and are in particular the refrigerating capacity, the coefficient of performance (or COP) and the condensation pressure.
The refrigerating capacity represents the refrigeration power available by virtue of the refrigerant, for a given compressor. In order to replace R-22, it is essential to have available a fluid having a high refrigerating capacity close to that of R-22.
The COP expresses the ratio of the refrigerating energy delivered to the energy applied to the compressor in order to compress the refrigerant in the vapor state. In the context of the substitution of R-22, a COP value of the refrigerant which is less than that of R-22 is suitable, if an increase in the consumption of electricity of the plant is accepted.
Finally, the condensation pressure indicates the stress exerted by the refrigerant on the corresponding mechanical parts of the refrigerating circuit. A refrigerant capable of replacing R-22 in a refrigeration system designed for the latter must not exhibit a condensation pressure significantly greater than that of R-22.
These novel HFC-based products, in particular R-407C, are not, however, compatible with the mineral oils or alkylbenzene oils used for systems operating with R-22 as regards the lubrication of mechanical parts, in particular because of an inadequate oil return. They thus require the use of novel oils of PolyOl Ester (POE) or PolyAlkylene Glycol (PAG) type.
The replacement, in the numerous existing heat transfer systems which were designed to operate with R-22, of the latter refrigerant by a refrigerant exhibiting a similar thermodynamic performance and an ozone depletion potential of zero thus requires, in addition to replacing the refrigerant, changing the lubricating oil, even changing certain components of the refrigerating circuit, such as the connecting pipe work and seals. Such a conversion procedure is virtually impossible with certain widely used items of compression equipment, such as the sealed compressor. It is in any case lengthy, difficult and expensive, all the more so as, in order to remove all the oil, it is necessary to rinse out several times with the new oil.
Furthermore, the document JP 8-100170 discloses a quaternary composition comprising difluoromethane, trifluoroethane, tetrafluoroethane and pentafluoroethane as replacement for R-502.